The present invention relates to an electric braking apparatus for alternating current electric motors, and is particularly adapted for braking such electric motors used for record turntables or the like.
Moden turntables for record player are known to have a relatively large mass and as a consequence braking means is necessary to bring the turntable to a quick stop. This is particularly the case for direct drive turntables. In such a turntable, driven by an a.c. electric motor, e.g., a single-phase induction motor, if there were no braking means the turntable might continue to rotate for up to three minutes after the power to the motor is cut off. This defect is particularly annoying when the operator desires to switch speeds, e.g., from 33 rpm to 45 rpm. In such cases a stabilized rotation will not be obtained quickly because of the large mass of the turntable.
Various methods and techniques are known for braking motors of the above mentioned type, and the most convenient are electric methods which do not require additional mechanical devices but do provide for automatic control of the braking process. One known technique is to reverse switch the windings of the a.c. motor thereby changing the phase sequence of the supply voltage to the windings and placing areverse torque on the rotor. In the latter case braking is controlled by a special speed control relay linked to the motor shaft. Unfortunately, such braking systems are not completely reliable due to the possibility of reverse rotation of the motor if the speed control relay fails.
The best known and most promising method of braking a.c. motors is dynamic braking, which is accomplished by feeding direct current (d.c.), via a suitable circuit, to one or more of the windings of the motor. The dynamic braking method is relatively smooth as compared with the reverse switching method, and has the further advantage that it eliminates the possibility of reverse rotation of the motor and provides for a braking moment practically during the whole process of braking. However, inasmuch as the d.c. current is produced by a half-wave rectifier, the current flowing through the windings will include a direct current component and an alternating-current component, thereby limiting the smoothness of the braking operation. Also, since such techniques often include the use of a mechanical relay circuit, switching noise or undesirable vibrations (e.g., mechanical chattering) may result.